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ancestors matured slowly One of our closest ancestors had more in common with gorillas than previously thought, with males of the species taking far longer to reach maturity than females, scientists say.

Males of Paranthropus robustus, an extinct relative of humans that lived almost 2 million years ago, continued to grow well into adulthood, before a lucky few finally established harems of females for breeding.

The result was a big difference in size between males and females.

"This is exactly what you see in gorillas and a variety of other primates, but not so much in humans or chimpanzees," says Dr Charles Lockwood of University College London, who publishes his study today in the journal Science.

Although modern human boys mature later than girls, the difference is small.

Male gorillas, by contrast, do not reach full dominant silverback status until many years after the females have already started to have offspring.

Experts believe the discoveries about P. robustus will help in understanding how modern human social structures have evolved and pinpoint how reproductive strategies diverged amongst early primates.

The findings, based on an examination of 35 fossilised specimens from South Africa, suggest that life was brutal for young P. robustus males, who faced intense competition as they fought with peers for the right to monopolise groups of females.

"Basically, males had a high-risk, high-return lifestyle in this species," Lockwood says.

"They most likely left their birth groups at about the time they reached maturity, and it was a long time before they were mature enough to attract females and establish a new group. Some of them were killed by predators before they got the chance."

Lockwood and colleagues established differences in size and maturity between different P. robustus specimens by measuring the size and shape of the skull.

The age of individuals was assessed by looking at how far their teeth had worn down.